Water and sediment discharge from small mountainous rivers, Taiwan: The roles of lithology, episodic events, and human activities

Taiwan's natural setting creates highly vulnerable watersheds whose rivers discharge disproportionately large quantities of sediment to the coastal ocean. The 16 Taiwanese rivers analyzed in this article discharge similar to 180 Mt yr(-1) of sediment to the coastal ocean, although totals over the past 20 years have varied between 16 and 440 Mt yr(-1). The mean annual sediment yield of 9500 t km(-2) yr(-1) for the 16 rivers is 60-fold greater than the global yield of 150 t km(-2) yr(-1), but mean yields for the individual rivers vary by more than 2 orders of magnitude, from 500 to 71; 000 t km(-2) yr(-1). Most sediment erosion and delivery occur in response to typhoon-generated floods, as evidenced by the fact that > 75% of the long-term flux occurs in < 1% of the time, about one-third of which reaches hyperpycnal concentrations. Detailed analysis of the 16 watersheds reveals little evidence of any single environmental factor that controls sediment load. The Erren, the highest-yield river on Taiwan, drains an erodible but low-gradient watershed with relatively low runoff. In contrast, three east coast rivers, the Hoping, the Hualien, and the Beinan, have high sediment yields that may be explained by relatively frequent earthquakes coupled with high runoff. Farming and urbanization also have elevated sediment yields in eastern watersheds, whereas Holocene sediments buried in the Taiwan Strait suggest that present-day sediment loads of the western rivers may be no higher than prehuman levels.